In order to be sold to the public, food often needs to be treated to minimize microbial growth that can occur between the time that the foodstuffs are harvested and they are purchased by the consumer. There are several general methods that are commercially available for this purpose, the most widespread of which is to heat the material to appropriate temperatures for sufficient lengths of time to kill or otherwise inactivate any microorganisms and/or spores that could germinate and grow at the storage temperature that may be present within the food. For example, milk is typically pasteurized in order to reduce the levels of bacteria that are normally found in the milk, which allows milk to be stored safely longer than it would be otherwise in the absence of the pasteurization process.
Generally, indirect heating methods are used in which the biomaterials are passed through a chamber that is heated to temperatures in excess of 60° C. for some heat sensitive pasteurizations to 100° C. and up to 150° C. to render materials commercially sterile. The presence of the biomaterials within the heated chambers results in the temperature of the biomaterials increasing until they reach substantially the same temperature as the surrounding chamber. However, many foodstuffs and other biomaterials are negatively impacted by the application of heat, either in terms of taste, aesthetic appearance, nutrient levels, or other characteristics so that the ways in which this material can be treated are limited. Additionally, many biomaterials exposed to a heated surface will burn on to the surface causing reduced heat flow, increased run times and can produce off flavors within the product as run time increases and heated material builds up and flakes off into the product.
For example, the utilization of sweet potatoes in the food industry often involves processing of the roots into purees that can be subsequently frozen or canned to allow year-round availability of the produce. The sweet potato puree (SPP) can be used as an ingredient in various products, including baby food, casseroles, puddings, pies, cakes, bread, restructured fries, patties, soups and beverages (Truong, 1992; Truong et al., 1995; Woolfe, 1992).
Preservation of SPP by freezing is a well-established method, but the frozen puree requires considerable investment in frozen distribution and storage as well as a lengthy and poorly controlled defrosting treatment prior to use. Canned puree typically requires excessive thermal treatment, especially when processed in institutional-size packages, provides poor utilization of storage space, and presents a difficulty in handling, opening, and dispensing of the product, as well as disposing of the emptied packages. Due to the poor heat penetration characteristic of the puree, canned sweet potatoes are retorted for over 2 hours at 121° C., resulting in product quality within a can that varies drastically from the can center to the wall edges. Particularly at the edges, the product is often severely over-processed, resulting in dark discoloration and burnt flavor. Thus, the useful can size is frequently limited to can size number 10 (i.e., a volume of about 13 cups), and this size limitation is a major obstruction to the wider applications of canned sweetpotato puree in the food processing industry.
Other thermal processing technologies such as scraped surface heat exchangers or flash sterilization treatment also have limitations in that SPP is characterized by low thermal diffusivity (Smith et al., 1982). Fasina et al. (2003) reported that SPP has a thermal diffusivity of the order of 3×10−7 m2/s and a thermal conductivity of the order of 0.54 W/m·K. The low thermal diffusivity of SPP leads to very long periods of heating when conventional thermal processing methods are used in order to achieve required sterilization levels, which in turn causes degradation of the nutrients in SPP and poor product quality.
Thus, there exists a long-felt and continuing need in the art for effective methods to thermally treat foods and other biomaterials. The presently disclosed subject matter addresses this and other needs in the art.